Corrosion under insulation (CUI) is corrosion that develops over time beneath thermal insulation used on pipes, tanks and other manufacturing and process equipment. Wherever piping, tanks or equipment are thermally insulated, there is potential for CUI. It is usually caused by condensation, rainwater, cleaning fluids, etc., that permeate into the insulation and attack the metal surface of the above equipment. Regardless of how securely insulation materials are applied to a substrate material, there will inevitably be areas where water can seep in, thereby creating conditions that subsequently cause corrosion and damage to the metal surface. As a consequence of the above, CUI may occur at any location and time, even at locations or under conditions which are initially regarded as less likely or even non-likely to experience corrosion.
Various publications are directed to insulation compositions or methods which aim at reducing the content of water in said insulation composition.
WO 2005/038330 described an insulation system, wherein a thermally insulating layer is arranged around a pipe. At the exterior of the insulating material a layer of hygroscopic material and a outer layer of a vapor barrier is present. JP2002181280 describes a method wherein through the insulation material wound around a pipe a gas of a temperature different from ordinary temperature in the inner part is passed. This gas picks up the water which is discharged at another point.
WO 91/18237 describes an insulation system comprising a layer of a heat insulating material for a conduit or container having a surface temperature below the dew point of the ambient air and in particular an insulating system for insulating cold pipes and conduits and containers for the transportation or storage of cooling media. The insulating system has layers of a hygroscopic wicking material on both sides of a thermally insulating material which is adapted to be arranged around a pipe. The two layers communicate with each other through an opening in the thermally insulating material, whereby condensate by capillary action can be transported from the inner layer to the outer layer. The wicking material is either directly applied onto the exterior of the cold pipe itself or a diffusion proof layer, such as a plastic film, is placed in direct contact with the exterior of the cold pipe. This plastic film should prevent condensed water from getting into direct contact with this exterior surface such to avoid corrosion.
WO 95/19523 describes an insulation around a pipe wherein strips of an hydroscopic material are equidistantly spaced from each other along the length of the pipe. The strips extend from a direct contact with the metal surface to the outside of the insulation where it is exposed to the ambient air and forms an evaporation surface.
A common feature of the prior-art solutions exemplified by WO 91/18237 and WO 95/19523 to the problem with removing of condensate is that a hygroscopic material is arranged on the metal surface on which condensate is formed. Another common feature is that the hygroscopic material is brought into direct contact with the ambient air.
A problem with the above insulation compositions is that either water is only removed locally in case strips are used. In case a wicking layer totally covers the metal layer corrosion may still take place, especially when the metal surface has a relatively high temperature. Such high temperature metal surfaces are for example encountered in transport conduits for steam. The use of a plastic film as described in WO 91/18237 will not avoid corrosion. This is because when such a film is damaged then liquid water will enter the space between the film and the metal pipe surface by means of capillary forces. Corrosion may then take place because this water cannot escape.